In vacuum furnace devices it is customary to have a hot zone within which the workpiece that is being "treated" is located. It is also customary in the fabrication of such vacuum furnaces that the hot zone be designed to provide an outside wall of heat insulating material so that the heat within the hot zone can be reasonably contained. Such walls of heat insulation material have more recently been made from graphite felt and graphite sheet with the later material being the inside layer of the composite heat insulating wall. Heat insulation walls formed of graphite felt and graphite sheet have been satisfactory for the most part, but such arrangements nonetheless do have infirmities. For instance, the initial cost of walls of graphite felt with graphite sheet inside layers is high. Secondly, graphite, if heated and exposed to oxygen, will burn. Thirdly, graphite interacts with materials which evaporate from various workpieces, or the graphite material cracks because of the difference in coefficients of temperature between built-up material and the basic graphite per se.
The present system uses ceramic oxide insulating panels, which material does not readily react chemically with materials from a workpiece. In addition, these ceramic oxide panels are capable of standing very high temperatures and temperature cycles without breaking or cracking and, thirdly, the cost of the ceramic oxide material is relatively low when compared with the cost of graphite.